add integration tests

[alejandro-vaz]

we should not rely exclusively on unit tests to ensure the correctness of the API

integration tests allow to catch exporting errors

I just missed one in #438 of AllocationError

probably the best way to do this is to move all unit tests in tests.rs to a tests/ folder and modularize there

[alejandro-vaz]

note: we also have to add testing and benchmarking for features

[alejandro-vaz]

@TDecking would you want to implement this??